The present invention generally relates to systems, equipment and methods for drying compressed gases, including but not limited to air.
The process of air compression concentrates atmospheric water vapor, resulting in condensation that can be harmful to processes and equipment that use compressed air and conduit systems (e.g., piping, tubing, etc.) that deliver the compressed air to those processes and equipment. For this reason, air dryers are widely used to remove water vapor from compressed air. Desiccant dryers are a well-known type of compressed air dryer used in a wide range of industrial and commercial facilities. Desiccant dryers typically operate to reduce the dew point of compressed air to at least about −40° C. (about −40° F.), which is sufficient to prevent water condensation under most conditions found in industrial and commercial facilities.
There are different types of desiccant dryers (also referred to as regenerative desiccant dryers or regenerative dryers), each characterized by certain advantages and drawbacks. However, desiccant dryers generally share in common the principle of using a desiccant to adsorb moisture from compressed air (or another gas) by passing the compressed air through a pressure vessel such as a tower or tank (hereinafter simply referred to as a “tank”) that contains a “bed” of suitable desiccant media, nonlimiting examples of which include activated alumina and silica gels. The desiccant material adsorbs water molecules from the compressed air, with the result that over time the desiccant bed eventually becomes saturated and can no longer effectively remove water from moisture-bearing compressed air entering the tank. To provide for continuous operation, a second tank of desiccant material is often provided so that the incoming compressed air can be switched between tanks when one tank becomes saturated. As one tank (hereinafter, the “in-service tank”) continues to dry the incoming moisture-bearing compressed air, the other tank (hereinafter, the “regenerating tank”) undergoes a “regeneration” process by which the moisture is purged from the desiccant material on which it was adsorbed.
One regeneration approach is to draw some of the dried compressed air from the in-service tank and use it to purge the desiccant bed undergoing regeneration in the regenerating tank. Typically, moisture-bearing compressed air undergoing drying in an in-service tank flows downward or upward through the desiccant in the tank, and dried compressed air used to remove moisture from the desiccant flows in the opposite direction through the regenerating tank in what is sometimes referred to as counter-flow regeneration. The moisture is effectively forced from the surfaces of the desiccant and removed at the lower end of the regenerating tank. In-service and regenerating tanks can be monitored to determine when regeneration should be performed on the in-service tank and regeneration has been completed on the regenerating tank, or regeneration can be performed on a time basis.
In what is sometimes referred to as “heatless drying,” the regeneration approach described above can be performed without heating the dried compressed “purge” air drawn from the in-service tank. Alternatively, the dried compressed purge air can be heated prior to being introduced into the regenerating tank to remove moisture from the desiccant within, after which the desiccant is typically cooled with unheated dried compressed air drawn from the in-service tank prior to returning the regenerating tank to in-service status. Though requiring the addition of a heater, this approach advantageously reduces the volume of dried compressed air needed to complete a regeneration cycle. Yet another alternative is to use heated atmospheric air as the purge air, after which the desiccant can be cooled with unheated dried compressed air drawn from the in-service tank. Though further requiring the addition of a heater to heat the purge air and a blower to force the purge air through the regenerating tank, a significant advantage of this approach is that it further reduces the volume of dried compressed air needed to complete the regeneration process.
Though the above regeneration approaches are well known and accepted in the art, it would be desirable if other approaches were available that might be capable of improving the efficiencies and/or reducing the cost of drying compressed air, particularly in industrial and commercial facilities.